In the railway industry, little has changed over the years in the methods of railway bridge construction. Since the beginning of railway bridge construction, vertical members (“piles”) were driven into the ground in successive rows across the width of a waterway or other geographic depression. Each row of piles typically contained two-six vertical piles made of timber. A horizontal timber member (“cap”) was then placed across the top of each row of timber piles, creating a series of “bents”, each bent comprising two-six vertical piles and a single horizontal cap. Horizontal timber members (“stringers”) were then placed to connect successive bents, creating the superstructure of the bridge. Finally, the road deck, cross ties, ballast, and rails were added to complete construction of the railway bridge.
Over the past 150 years, however, these bridges have deteriorated to the point that they have been rebuilt several times over the course of the years. Initially, the bridges were repaired by driving new timber pile bents between the existing bents, and then replacing the timber stringers to span the new bents. The older bents were then removed by simply cutting their piles at ground level, leaving a substantial portion of the old pile stubs still in the ground.
This process would be repeated several times over the decades, eventually leaving a congested area beneath the bridge full of stubs of old piles. Eventually, the area beneath the bridge became so congested with the stubs of old piles that this method could no longer be used without removing the pile stubs at significant cost to the railroad.
Subsequently, modern replacement methods were developed, typically involving the use of a single pair of steel piles per replacement bent, each pile being driven into the ground on either side of the congested area immediately beneath the existing bridge. Once these steel piles were driven into the ground and reinforced with steel and concrete, the engineers would use cast-in-place construction techniques to cast a concrete cap atop the pair of driven steel piles. Typically, the engineers would begin this cast-in-place technique by placing a cap form around the tops of each pair of driven piles. Next, the engineers would position reinforcing bars (“rebar”) inside the cap form. Finally, the engineers would pour concrete into the form and allow it to cure.
Further, to minimize the time period for disrupting traffic over an existing bridge, such replacement bents were typically built at a height slightly lower than the existing bridge. Thus, the substructure of the replacement bridge could be built while rail traffic still flowed over the existing bridge. Once the replacement bridge substructure was complete, traffic would be stopped on the rail line. The old bridge would then be dismantled, new spans would be placed atop the new bents, and the approaches to the old bridge would be modified so the rail line could use the new bridge.
This method of bridge repair has certain drawbacks, however. First, the method is quite time consuming and expensive because the caps for the replacement bridge must be carefully cast, in situ, without damaging the existing bridge or disrupting the traffic traveling over the existing bridge. Also, the concrete in the caps must be given time to cure before the caps can support loads and the replacement bridge can be completed. Furthermore, the practice of casting the caps at the worksite necessitates the use of local concrete and reinforcing materials, the quality of which is variable from one concrete plant to the next.
This method also has the drawback that the replacement bridge must be placed at a lower elevation than the existing bridge because the replacement bridge must be built beneath the existing bridge to allow rail traffic to flow during construction. The lower elevation of the replacement bridge reduces the clearance between the replacement bridge and an underlying waterway, thus potentially interfering with shipping and increasing the likelihood that the replacement bridge may be affected by flooding. The lower replacement bridge elevation may also necessitate that additional building permits be obtained and/or environmental impact studies be conducted.